1. Field of Invention
Embodiments of the invention generally pertain to electronically-steered pulse-Doppler radar systems and methods. More particularly, embodiments on the invention pertain to a method of radar energy management and Doppler processing. Embodiments are further directed to a method for artificially increasing the coherent integration time (Doppler resolution) for a given receive position without penalty to the overall radar timeline.
2. Description of Related Art
Doppler processing in radar systems is well known. Referring to FIG. 1, in a typical modern pulsed Doppler radar, a set number of multiple pulses are transmitted at a constant “pulse repetition interval” (PRI). The total number of pulses to be processed at a time multiplied by the PRI is typically referred to as the “coherent period of integration” (CPI). The radar processor stores received complex signals (Range samples) as a function of time (sometimes referred to as “fast time,” corresponding to samples in range of energy returned from a single pulse) from each successive individual pulse (sometimes referred to as “slow time,” with the value corresponding to pulse or pulse repetition interval (PRI) number within a given CPI). Using data corresponding to common points in range (i.e., a “fast time” sample of the same range) of the PRIs within a CPI, a Fourier transform is performed across the data points for all of the PRIs within a CPI. This calculation transforms phase change rate across the PRIs at a common range into Doppler. When performed for all range samples, the result is a two dimensional array of complex values that correspond to radar Range and Doppler.
The “Doppler resolution” is often described as the ability to separate two sources of response at two different Dopplers at the same range. For pulsed-Doppler radars, the Doppler resolution is a function of radar frequency and the CPI.
“Unambiguous Doppler” corresponds to the breadth of range rate that the radar can measure a target unambiguously, i.e. without aliasing. The unambiguous Doppler is determined by the radar frequency and the radar “pulse repetition frequency” (PRF). The PRF can be represented as the inverse of the PRI duration. As such, the unambiguous Doppler divided by the number of PRIs within a CPI results in a value that approximates the Doppler resolution capability. For example, in the case that the frequency and CPI duration result in an unambiguous Doppler capability covering 0 meters per second (m/s) to 320 m/s, and the CPI contains 32 pulses, the resulting Doppler resolution can be approximated to be 10 m/s.
Classical pulse-Doppler type radar systems were mechanically scanned, and utilized simple processing. Subsequent ESA (Electronically Scanned Array) radar systems were typically used for long range operation. Both of these types of radar systems utilized consecutive pulses at a common angle location to create a CPI and perform Doppler processing. With the advent of modern, low cost, high-transmit duty cycle, short range ESA radar systems that are capable of precision radar beam pointing at extremely rapid rates, such as the AN/TPQ-48 radar system developed by SRC, Inc., additional degrees of freedom in managing radar energy have required new techniques in Doppler processing.